The risks of early and late bleeding, placenta previa, abruptio placentae, and premature and prolonged rupture of membrane increase with increasing levels of maternal smoking during pregnancy. However, the degree of risk appears to vary depending on other maternal factors. The objectives of this study are: 1) to identify women in terms of age-parity, socioeconomic status, height, weight, weight gain, hemoglobin level, past obstetric and medical history, use of medical services, and other factors who will be at highest risk for these complications if they smoke; 2) to trace the progression of these complications prospectively to preterm or term delivery and to perinatal death or survival, weighing the relative importance of maternal smoking and of other risk factors in the process; 3) to serve as a model for future prospective studies by identifying mothers at maximum risk of complications and by determining an optimum comparison group. These objectives will be carried out using data from the Ontario Perinatal Mortality Study of 1960-61. The 31,788 Canadian born, English-speaking mother-infant pairs will be the population source for a series of studies. Women with specified complications will be compared retrospectively with several comparison groups and will then be followed as if prospectively to pregnancy termination. Comparison groups without complications will be group-matched by time of registration and by hospital pay status, with and without selected other factors. Crosstabulations and multiple regression analysis will determine the relative importance of each risk factor independently and will measure interactions between smoking and other risk factors in relation to complication rates and to adverse or successful pregnancy outcomes. The large size of the study population, the variety of available items and the readiness of the tape to be used make it possible to carry out these procedures at a high level of efficiency.